ma s-08-657131 May 13, 2021 Time: 13:57 # 1
BRIEF RESEARCH REPORT
published: 14 May 2021
doi: 10.3389/ ma s.2021.657131
Edi ed by:
Kyung-Hoon Shin,
Hanyang Uni e si y, Sou h Ko ea
Re iewed by:
Ra ik Bal i,
Uni e si y o Jendouba, Tunisia
Tieyu Wang,
Shan ou Uni e si y, China
*Co espondence:
An onio Bode
[email p o ec ed]
Special y sec ion:
This a icle was submi ed o
Ma ine Pollu ion,
a sec ion o he jou nal
F on ie s in Ma ine Science
Recei ed: 22 Janua y 2021
Accep ed: 19 Ap il 2021
Published: 14 May 2021
Ci a ion:
Bode A, Besada V,
Pé ez-Fe nández B and Viñas L
(2021) Amino Acid δ15N Can De ec
Die E ec s on Pollu ion Risks o
Yellow-Legged Gulls O e looked by
T ophic Posi ion.
F on . Ma . Sci. 8:657131.
doi: 10.3389/ ma s.2021.657131
Amino Acid δ15N Can De ec Die
E ec s on Pollu ion Risks o
Yellow-Legged Gulls O e looked by
T ophic Posi ion
An onio Bode1*, Vic o ia Besada2, Begoña Pé ez-Fe nández2and Lucía Viñas2
1Ins i u o Español de Oceanog a ía, Cen o Oceanog á ico de A Co uña, A Co uña, Spain, 2Ins i u o Español de
Oceanog a ía, Cen o Oceanog á ico de Vigo, Vigo, Spain
The use o op-consume s as bioindica o s o he heal h o ood webs is hampe ed
by unce ain ies in hei e ec i e use o esou ces. In his s udy, he abundance o
s able ni ogen iso opes in amino acids om homogenised eggs o he Yellow-legged
Gull (La us michahellis) allowed o iden i y a ia ions in ophic esou ce exploi a ion
be ween geog aphically adjacen nes ing colonies in he Ria de Vigo (NW Spain) ha
exhibi ed ma ked di e ences in pollu an s. Eggs om nes s in he Cíes Islands (loca ed
in a Na ional Pa k) showed a la ge a iabili y in s able ca bon and ni ogen iso opes in
bulk egg con en encompassing ha o eggs om Vigo ci y (a majo ishing ha bou ).
Howe e , bo h colonies di e ed in he ela i e concen a ion and abundance o ni ogen
iso opes o lysine, an essen ial amino acid p esen in ma ine p ey, bu also ex ensi ely
used in eed s ocks o poul y and swine. No wi hs anding he simila i y in ophic
posi ion o bo h colonies, gulls om Cíes Islands may ha e acqui ed a subs an ial
ac ion o lysine om ga bage dump si es, while hose o he u ban colony elied on ish
disca ds. This unexpec ed conclusion is pa ly suppo ed by he la ge a iabili y epo ed
o gull’s die in his egion and calls o de ailed es ima ions o die when assessing he
conse a ion s a us and pollu ion isks o ma ine ecosys ems.
Keywo ds: bioindica o , s able iso opes, amino acids, seagull eggs, ood web
INTRODUCTION
Top consume s om oceanic ood webs a e good indica o s o he biogeochemis y and
conse a ion s a us o ma ine ecosys ems. They in eg a e p ocesses ope a ing a lowe ood web
le els and hus ep esen he esul o ecosys em unc ioning a la ge spa ial and empo al scales
(Hebe e al., 2016;Ruiz-Cooley e al., 2017). Es ima ions o hei ophic posi ion (TP) a e key
o de e mine ecosys em p ope ies as ood-chain leng h (Vande Zanden and Fe ze , 2007;Reum
e al., 2015) o he biomagni ica ion o pollu an s (S o -Hansen e al., 1995;Die z e al., 2000;
Kelly e al., 2008). Fo ins ance, changes in TP a e indica i e o al e a ions in ood web s uc u e
(Jenkins and Da o en, 2020) o in nu ien inpu s (Ruiz-Cooley e al., 2017). Pa icula ly, seabi ds
ha e been ex ensi ely used as biomoni o s o pe sis en o ganic and ace me al con aminan s (e.g.,
OSPAR, 1999), wi h biomagni ica ion e ec s c i ically dependen on hei TP (Ramos e al., 2013;
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Bode e al. Amino Acid δ15N in Gulls
Ga e al., 2020). Me hodological unce ain ies, howe e , di icul
he accu a e de e mina ion o TP in hese o ganisms.
Di ec quan i ica ion o die s canno be achie ed o mos ield
popula ions, as hey may be inaccessible o long pe iods (e.g.,
Ma ias and Ca y, 2010;Ga e al., 2020) and he analysis o
s omach con en s, pelle s o egu gi a es canno iden i y all he
consumed p ey (González-Solís, 2003;Abdennadhe e al., 2010;
Mo eno e al., 2010;Calado e al., 2018;Méndez e al., 2020).
Al e na i e de e mina ions based on he p og essi e en ichmen
in hea y ni ogen iso opes (exp essed as δ15N) h ough he
ood web allow o es ima ions o TP and die composi ion
in eg a ed in space and ime, bu a e highly dependen on ac o s
as he app op ia e selec ion o e e ence baselines (Pos , 2002),
he assump ions o he en ichmen model (Hussey e al., 2014;
Jennings and an de Molen, 2015) and he issue analysed (Cau
e al., 2009;Hebe e al., 2016). Ne e heless, hese limi a ions
ha e no impeded he ex ensi e use o δ15N in bulk issues o
in e TP and biomagni ica ion o pollu an s in many s udies (e.g.,
Abdennadhe e al., 2010;Ramos e al., 2013). The applica ion
o compound-speci ic iso opic analyses (e.g., δ15N in amino
acids) ha e educed some o he unce ain ies in TP es ima ions,
because he changes in he nu ien sou ces a he base o he
ood web can be accoun ed o Chika aishi e al. (2009). E en
when he e a e s ill unce ain ies in he a iabili y o he iso opic
en ichmen along he ood web (McMahon and McCa hy, 2016;
Ohkouchi e al., 2017;Whi eman e al., 2019) he new es ima ions
based on amino acids a e being inc easingly applied o ma ine
bi ds (McMahon e al., 2015;Wu e al., 2018;Ga e al., 2020).
Eggs o he Yellow-legged Gull (La us michahellis) ha e
been used o moni o o ganic and ace me al pollu an s
(Abdennadhe e al., 2010;Ramos e al., 2013;O e o e al., 2018;
Viñas e al., 2020). The la ge eeding plas ici y epo ed o his
species, howe e , di icul he iden i ica ion o he sou ces o
pollu an s. Fo ins ance, gull colonies nea u ban a eas ha e been
shown o consume mo e e es ial and an h opogenic esou ces
han ma ine p ey (A izaga e al., 2013;Méndez e al., 2020;
Zo ozua e al., 2020). A p e ious s udy o wo colonies only
10 km apa in he Ria de Vigo (NW Spain) e ealed signi ican
di e ences in he load o pollu an s in hei eggs, poin ing o a
apid adap a ion o local eeding esou ces (Viñas e al., 2020).
The colony nes ing on he Cíes Islands (loca ed in a ma ine
p o ec ed a ea) showed lowe concen a ions o pollu an s and a
wide ophic niche han he colony nes ing in he indus ialised
Vigo ci y. Fu he mo e, he es ima ed die composi ion in he
egion, including colonies in he Cíes Islands, showed ma ked
yea o yea a ia ions ela ed o he a ailabili y o ma ine s.
an h opogenic esou ces, he la e including mainly bee , po k,
and chicken emains (Mo eno e al., 2010;Calado e al., 2020).
These indings sugges ha majo changes in he die o he
Yellow-legged Gull may a ec TP and he e o e TP can be
used as an indica o o changes in he ophic s uc u e o he
ood web. Al e na i ely, pollu ion loads could be caused by he
di e en ial exploi a ion o ophic esou ces wi hou a subs an ial
change in TP.
The objec i e o his s udy is o de e mine he ophic
posi ion o Yellow-legged Gull in wo colonies di e ing in he
accumula ion o pollu an s (Viñas e al., 2020). We applied o
he i s ime es ima ions de i ed om he amino acid s able
iso ope composi ion o eggs, allowing o in e di e ences in
ophic posi ion and die be ween he gulls in he wo colonies.
These es ima ions aimed o assis in he in e p e a ion o he
di e en ial impac o con aminan s in hese colonies by e ealing
he sensi i i y o TP es ima ions o die a iabili y.
MATERIALS AND METHODS
Sampling
Yellow-legged Gull eggs we e collec ed om nes s in he Cíes
Islands (11 eggs) and in he ci y o Vigo (7 eggs) du ing
he b eeding season o 2011 (Ap il o ea ly Augus ). Fi s
laid eggs in each nes we e selec ed o analysis, weighed and
measu ed (leng h and wid h) be o e opening (Supplemen a y
Table 1). Emb yona ed eggs we e disca ded and he emaining
we e indi idually homogenised, s o ed ozen (−20◦C), and
eeze-d ied be o e analysis. Fu he de ails on sampling we e
p o ided in Viñas e al. (2020).
Analysis
Two ypes o iso opic de e mina ions we e made. Fi s , C:N a ios
and na u al abundances o ca bon (δ13C) and ni ogen (δ15N)
s able iso opes we e de e mined in bulk egg samples using an
iso ope a io mass spec ome e coupled o an elemen al analyze .
Egg samples we e analysed in iplica e wi h p ecision <0.05h
o bo h iso opes. Due o he high lipid con en o egg samples,
δ13C alues we e co ec ed using he C:N a io o each sample
TABLE 1 | Mean and sd ophic posi ion alues (TP) es ima ed o gull species
using s able iso opes in his s udy and in he li e a u e.
Species Mean SD nZone Re e ences
La us michahellis13.96 0.67 18 Galicia (NE A lan ic) This s udy
La us michahellis23.86 0.47 18 Galicia (NE A lan ic) This s udy
La us michahellis33.70 0.42 60 Tunis
(Medi e anean
Sea)
Abdennadhe
e al., 2010
La us michahellis43.91 1.56 30 S Po ugal (NE
A lan ic)
Lopezosa e al.,
2019
La us a lan icus33.20 0.32 11 Pa agonia (SW
A lan ic)
Fo e o e al.,
2004
La us dominicanus33.80 0.14 229 Pa agonia (SW
A lan ic)
Fo e o e al.,
2004
La us a gen a us33.93 0.51 187 G ea Lakes Hebe e al.,
1999
La us a gen a us33.29 0.25 16 Bal ic Sea Sø mo e al.,
2011
La us a gen a us53.60 0.40 4 Lake Hu on Hebe e al.,
2016
N, numbe o da a. De ails on he es ima ion models a e gi en in he sec ion
“Ma e ials and Me hods.”
1δ15Nbulk in eggs, one baseline, SD = p opaga ed e o .
2δ15NAA in eggs, 2 TDF, SD = p opaga ed e o .
3δ15Nbulk in ea he s, one baseline; SD = epo ed SD.
4Calcula ed om o iginal δ15Nbulk in ea he s and die da a, 3 baselines (pelagic
ish, deme sal ish, e es ial sou ces), SD = p opaga ed e o .
5δ15NAA in muscle, 2 TDF ( e es ial, aqua ic), SD = epo ed SD.
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Bode e al. Amino Acid δ15N in Gulls
(Ellio and Ellio , 2016). These analyses we e desc ibed in mo e
de ail in Viñas e al. (2020).
Second, 10 mg aliquo s o he homogenised eggs we e also
used o de e mina ions o δ15N in indi idual amino acids
(Chika aishi e al., 2009). Samples we e hyd olysed wi h 6N HCl,
es e i ied wi h ace yl chlo ide:2-p opanol, and ea ed wi h a
mix u e o 3:1 diclome hane: i luo ace ic anhyd ide (McCa hy
e al., 2013;Mompeán e al., 2016). De i a ised amino acids
we e pu i ied by sol en ex ac ion in 1:2 chlo o o m:phospha e
bu e and cen i uga ion (Loick e al., 2019), e apo a ed a
oom empe a u e unde N2, and s o ed a −20◦C in 3:1
diclome hane: i luo ace ic anhyd ide o up o 6 mon hs un il
iso ope analysis. The indi idual amino acids we e sepa a ed in a
ch oma og aphy column (T aceGOLD TG-5MS, 60 m, 0.32 mm
ID, 1.0 µm ilm) using a gas ch oma og aph (T ace1310GC,
The mo Fishe Scien i ic), and subsequen ly injec ed in o a
mass spec ome e (Del aV Ad an age, The mo Fishe Scien i ic)
ia a con inuous low in e ace and combus ion module (GC
Isolink, The mo Fishe Scien i ic). Amino acid δ15N alues we e
calib a ed wi h he alues ob ained o isola ed s anda ds (Shoko
Science) analysed by combus ion as desc ibed o bulk analysis,
and u he co ec ed using an in e nal L-no leucine s anda d
(SIGMA) o known iso opic composi ion added o each sample.
The mola ac ion o each amino acid (% mola ) was also
de e mined in he same analy ical un by calib a ion o he
spec ome ic signals wi h amino acid s anda ds (McCa hy e al.,
2013). P ecision (±SE) o iplica e samples ( wo injec ions pe
sample) was <0.6hpe indi idual amino acid.
Values o δ15N and %mola we e ob ained o ophic and
sou ce amino acids (McClelland and Mon oya, 2002;McCa hy
e al., 2013;McMahon and McCa hy, 2016). T ophic amino
acids included alanine (Ala), leucine (Leu), isoleucine (Ile),
p oline (P o), aline (Val), and he mix u es o glu amine
(Gln) and glu amic acid (Glu), and o aspa amine (Asn) and
aspa ic acid (Asp). The la e mix u es we e caused by he
acid hyd olysis and we e e med as Glx and Asx, espec i ely.
Sou ce amino acids included glycine (Gly), h eonine (Th ),
se ine (Se ), me hionine (Me ), phenylalanine (Phe), and lysine
FIGURE 1 | Box plo o (A) δ15Nbulk,(B) lipid-co ec ed δ13Cbulk ,(C) δ15N p, and (D) δ15Ns c o Yellow-legged Gull eggs sampled in Cíes Islands (black) and Vigo
( ed). Each box encompasses he 25–75% pe cen iles, he median is indica ed by he ho izon al line, and he whiske s indica e he ange excluding ou lie s
exceeding 1.5 (ci cles) o 3 imes (as e isks) he in e qua ile ange.
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Bode e al. Amino Acid δ15N in Gulls
(Lys). The ep esen a ion o hese amino acids in he bulk p o ein
was assessed by co ela ion o hei mola -weigh ed a e age
δ15N (δ15NTHAA) wi h δ15N alues o bulk samples (δ15Nbulk).
Simila ly, he co ela ions be ween mola -weigh ed a e ages o
ophic (δ15N p) o sou ce amino acids (δ15Ns c) wi h δ15Nbulk
we e used o de e mine he ela i e impo ance o a ia ions
in ophic s. ni ogen sou ce ac o s o he in e p e a ion o
δ15Nbulk.
Die Es ima ions
The po en ial con ibu ion o ma ine and an h opogenic ood
sou ces o he die o he gulls o each colony was es ima ed om
he δ15N signa u e o sou ce amino acids. Re e ence alues o
ma ine sou ces we e p o ided by sa dines (Sa dina pilcha dus)
and ancho ies (Eng aulis enc asicholus) collec ed in sp ing (Ap il
2017 and Ap il 2019) nea he Ria de Vigo as pa o a egula
ish su ey (see Bode e al., 2018). An h opogenic sou ces we e
p o ided by samples o bee , po k and chicken mea ob ained
om local ma ke s. Fou indi iduals o each ish species and h ee
eplica es o each mea ype we e analysed as desc ibed o he
gull eggs. The po en ial con ibu ion o hese sou ces o he gulls
die we e es ima ed using he Mix SIAR Bayesian mixing model
(SIAR 4.1.3) o S ock and Semmens (2013) by using only he
δ15N alues o indi idual amino acids ha a ied signi ican ly
be ween colonies and assuming no iso opic ac iona ion be ween
sou ces and eggs. This simpli ied app oach was in ended only o
highligh he po en ial implica ions o he ma ked di e ences in
δ15N among he sou ces.
T ophic Posi ion
Es ima ions o he ophic posi ion o Yellow-legged Gull
indi iduals we e made using he δ15N alues o ep esen a i e
ophic (Glx) and sou ce (Phe) amino acids (TPAA) and a model
conside ing wo ophic disc imina ion ac o s (McMahon and
McCa hy, 2016):
TPAA =2+(δ15NGlx−δ15NPhe−TDFp−β
TDFb
)(1)
Whe e, βis he di e ence be ween Glx and Phe in p ima y
p oduce s, and TDFp and TDFb a e he ophic disc imina ion
ac o s o plank on and ma ine bi ds, espec i ely. Mean (±SE)
alues we e 3.4 ±0.9h o β(Chika aishi e al., 2009),
FIGURE 2 | Mean ±SE (A) δ15N (h) and (B) mola pe cen o ophic and sou ce amino acids o Yellow-legged Gull eggs sampled in Cíes Islands (black do s) and
Vigo ( ed do s). The a ows indica e signi ican ly di e en medians (K uskal-Wallis, P<0.05).
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Bode e al. Amino Acid δ15N in Gulls
7.6 ±1.2h o TDFp and 3.5 ±0.4h o TDFb (McMahon e al.,
2015;Wu e al., 2018;Ga e al., 2020). As he e was iso opic
ac iona ion be ween he di e en issues, inc emen s o 2.5h
and 0.9hwe e applied o he measu ed alues o δ15NGlx and
δ15NPhe, espec i ely, o p oduce TP es ima ions equi alen o
hose ob ained o he muscle o adul gulls (Hebe e al., 2016).
Fo compa a i e pu poses, addi ional TP es ima ions we e
made using he measu ed δ15Nbulk in eggs using he classical
model (Pos , 2002):
TPbulk =2+(δ15Nbulk−δ15Nzoo
TDFz
)
whe e, δ15Nzoo was he alue o zooplank on samples
collec ed in he nea by shel du ing sp ing 2011
(mean ±se = 5.97 ±0.92h,n= 14) as desc ibed in Bode
e al. (2018), and TDFz= 3.4h(Pos , 2002). The e o o all TP
es ima ions was compu ed by e o p opaga ion by aking in o
accoun he analy ical e o s in δ15N measu emen s in bulk o
ophic and sou ce amino acids, as well as he e o s epo ed o
TDF and β alues employed (Pos , 2002;Ohkouchi e al., 2017).
Fu he compa isons o TP we e made wi h hose epo ed
in he li e a u e o Yellow-legged Gull and simila species
(Table 1). In he case o da a epo ed in Lopezosa e al.
(2019),TP was es ima ed om he epo ed δ15Nbulk in body
ea he s and die da a using he model desc ibed in Hebe e al.
(2016) by conside ing he consump ion o pelagic ish, deme sal
ish, and ga bage.
S a is ics
Di e ences in iso opic measu emen s be ween eggs om Cíes
and Vigo colonies we e analysed by means o non-pa ame ic
ANOVA (K uskal-Wallis). Reg essions be ween a iables we e
compu ed using educed majo axis eg ession, as he e we e
la ge di e ences in he measu emen e o o he a iables.
S a is ical analyses we e pe o med using SPSS 17.0 (SPSS Inc.)
and Pas 4.0 (Hamme e al., 2001).
RESULTS
The e we e no signi ican di e ences in median bulk δ15N o
δ13C be ween bo h colonies, no in a e aged δ15N alues o
ophic o sou ce amino acids (K uskal-Wallis es , P>0.05,
Figu e 1). Howe e , he weigh ed means o δ15N o o al
amino acids (δ15NTHAA) and ophic amino acids (δ15N p) we e
linea ly co ela ed wi h bulk δ15N (P<0.001, n= 16), while he
means o sou ce amino acids showed no co ela ion (P>0.05,
Supplemen a y Figu e 1).
Values o δ15N alues o mos amino acids we e simila o
he wo colonies (Figu e 2A), excep in he case o lysine ha
showed lowe alues o he Vigo colony (K uskal-Wallis es ,
P<0.05). Mean ±SD δ15N alues o lysine in eggs om he
Cíes Islands we e 1.50 ±1.18h(n= 9) while in hose om Vigo
we e 4.68 ±0.77h(n= 7). E en when adjus ed o a ia ions
in he δ15N o he canonical sou ce amino acid phenylalanine,
he di e ences in iso opic composi ion o Lys be ween colonies
pe sis ed, and we e indica i e o di e ences in he use o ma ine
FIGURE 3 | Box plo o ophic posi ion o Yellow-legged Gull es ima ed om
(A) bulk δ15No (B) amino acid δ15N o samples om Cíes Islands (black)
and Vigo ( ed). Boxes and symbols as in Figu e 1.
esou ces (Supplemen a y Table 2). The eggs om he Cíes
Islands had also lowe mean ela i e mola concen a ion o
lysine and highe concen a ion o alanine han hose om Vigo
(K uskal-Wallis es s, P<0.05, Figu e 2B).
As o gull eggs, he e we e also signi ican di e ences in
Lys δ15N among ood sou ces (Mann-Whi ney es , P<0.05,
Supplemen a y Figu e 2A). Using hese alues, he es ima ions
o die sugges ed ha , no wi hs anding ma ine sou ces had a
majo con ibu ion po en ial o bo h colonies, he gulls om
Cies had a highe con ibu ion o an h opogenic sou ces han
hose om Vigo (Supplemen a y Figu e 2B).
The es ima ions o TP (Table 1) indica ed ha he s udied
Yellow-legged Gulls we e almos e ia y consume s (i.e., TP ca.
3) and e ealed no signi ican di e ences using ei he bulk o
amino acid δ15N (K uskal-Wallis es , P>0.05). Besides, he e
we e no signi ican di e ences in TP be ween colonies (Figu e 3,
K uskal-Wallis es , P>0.05).
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Bode e al. Amino Acid δ15N in Gulls
DISCUSSION
The esul s o his s udy ep esen he i s es ima ion o
Yellow-legged Gulls TP ob ained wi h amino acid δ15N. These
es ima ions a e compa able o hose ob ained using bulk δ15N
in his and p e ious s udies in he same and ela ed gull
species (Table 1), and in o he ma ine bi ds (Ga e al., 2020).
Fu he mo e, he signi ican co ela ion be ween δ15N o ophic
amino acids and bulk δ15N (and he lack o co ela ion o δ15N
in sou ce amino acids) suppo s p e ious assump ions using bulk
δ15N as an index o TP (Abdennadhe e al., 2010;Muñoz-A nanz
e al., 2012;Ped o e al., 2013;Calado e al., 2018). Howe e ,
he de e mina ion o δ15N in indi idual amino acids p o ided
addi ional in o ma ion on he a iabili y o eeding among
indi iduals ha complemen s TP es ima ions. Fo ins ance, he
simila i y in TP be ween he s udied colonies suppo s ha he
di e ences in pollu ion loads epo ed in Viñas e al. (2020)
we e caused by changes in die un ela ed o TP, e en when
de ailed die composi ion was no a ailable o he indi iduals
s udied. Simila conclusions we e eached in o he s udies whe e,
die and TP could be de e mined (Muñoz-A nanz e al., 2012;
Ramos e al., 2013).
The low δ15N in lysine ound in eggs om he Cíes Island
colony sugges he consump ion o non-ma ine p ey. Fi s ,
because he mean di e ence be ween δ15N o Lys and Phe
o he Cíes eggs was signi ican ly lowe han alues ypical
o phy oplank on (Nielsen e al., 2015). Second, because he
es ima ed con ibu ions o an h opogenic sou ces o he die in
Cíes we e gene ally highe han hose in Vigo (Supplemen a y
Figu e 2). This is an unexpec ed esul , as he gulls om
he islands would ha e, in p inciple, mo e access o ma ine
p ey han hose nes ing in he ci y. Lysine is an essen ial
amino acid in he die o animals and is one o he majo
addi i es o eed s ocks, no ably o poul y and swine aising
(To ide, 2002). I is p oduced indus ially h ough e men a ion
wi h he addi ion o suga s and ino ganic ni ogen (Ikeda,
2017). Indus ial lysine is likely o ha e low δ15N, since
ammonium de i ed om a mosphe ic ni ogen ep esen s he
mos economic sou ce o e ilize s (Smil, 2001). Fo ins ance,
he alues o Lys δ15N measu ed in bee , po k and chicken
mea in his s udy we e lowe han hose measu ed in ish.
These esul s ag ee wi h hose epo ed o bulk δ15N om
chick gull egu gi a es in he Bay o Biscay (A izaga e al.,
2013). The e o e, a subs an ial consump ion o ea ing animals
emains by gulls om he Cíes Island colony canno be
disca ded, as no ed by p e ious s udies (Mo eno e al., 2010).
Gi en he la ge a iabili y epo ed in he die o he Yellow-
legged Gull, i would be no su p ising ha he Cíes Islands
colony pa ly elied on ga bage, a leas du ing he ime ame
conside ed in his s udy. This is suppo ed by he yea - o-
yea educ ion in he consump ion o ish by his species
along he NW Spanish coas and a ibu ed o he decline
in sa dine popula ions (Calado e al., 2020). Fu he mo e, he
epo ed mean composi ion o he gull die in his egion
indica ed he dominance o an h opogenic o e ma ine ood
i ems du ing 2010, an excep ional con ibu ion compa ed o
he se ies o he whole eco ding pe iod (Supplemen a y
Table 3). In his ega d, i mus be no ed ha he iso opic
composi ion o eggs is conside ed ep esen a i e o bo h ecen
(ca. 1 mon h) and pas (ca. 1 yea ) die o he b eeding
adul emales (Hebe e al., 2016). The high consump ion
o ma ine esou ces in Vigo ag ees wi h he gene al pa e n
epo ed o he egion and could be a ou ed by he p oximi y
o he ishing ha bou , one o he la ges in Eu ope wi h
84,000 me ic ons o esh ma ine p oduc s landed in 2011
(Po Au ho i y o Vigo, 2012). Fish o al would be a a ou ed
esou ce o gulls nes ing in Vigo, as epo ed o simila
species elsewhe e (Hebe e al., 1999;González-Solís, 2003;
Calado e al., 2018).
Di e ences in die , hus, we e he likely cause o he high
le els o pollu an s in eggs om he Vigo colony (Viñas e al.,
2020). Ma ine ishes o high TP we e epo ed o bioaccumula e
o ganic and ino ganic pollu an s. Fo ins ance, Hg compounds
in una (e.g., Méndez e al., 2001) can each concen a ions
equi alen o hose ound in he gull eggs om Vigo (Viñas e al.,
2020). These esul s imply ha biomagni ica ion does no only
depend on TP bu also on he die o he consume species.
The e o e, u u e assessmen s o pollu ion isks in ood webs
would equi e coupled s udies o die and TP. Gull die can be
econs uc ed om obse a ions o eeding emains o s omach
con en s, bu also om ins umen al echniques as he analysis
o ma ke a y acids (Hebe e al., 2016). The la e can be e y
sensi i e when coupled o s able iso ope de e mina ions (Twining
e al., 2020). The applica ion o hese echniques will help o
de e mine he app op ia e biomagni ica ion ac o s when he
moni o ed species, as he Yellow-legged Gull, eeds on di e en
habi a s o ood webs.
Conclusion
Ou s udy shows ha eliable TP es ima ions o he yellow-
legged gull can be ob ained using δ15N ei he in bulk
homogenised eggs o in indi idual sou ce and ophic amino
acids. Howe e , he la e echnique would be p e e ed because
da a o assump ions abou he baseline e e ence alues om
o he species a e no equi ed, and because i can p o ide
aluable in o ma ion on he a iabili y in die among indi iduals.
Indeed, die , a he han TP, appea s as he dominan ac o
a ec ing he bioaccumula ion o pollu an s in he Yellow-
legged Gull.
DATA AVAILABILITY STATEMENT
The o iginal con ibu ions p esen ed in he s udy a e included
in he a icle/Supplemen a y Ma e ial, u he inqui ies can be
di ec ed o he co esponding au ho .
ETHICS STATEMENT
E hical e iew and app o al was no equi ed o he animal s udy
because gull eggs o his s udy we e collec ed as pa o he
F on ie s in Ma ine Science | www. on ie sin.o g 6May 2021 | Volume 8 | A icle 657131
ma s-08-657131 May 13, 2021 Time: 13:57 # 7
Bode e al. Amino Acid δ15N in Gulls
pollu ion moni o ing p og am o he IEO and he Minis e io pa a
la T ansición Ecológica (Spain).
AUTHOR CONTRIBUTIONS
LV and VB designed he p ojec . AB designed he esea ch
epo ed he e, analysed he da a, and w o e he manusc ip
wi h commen s om coau ho s. LV, BP-F, and AB analysed he
samples. All au ho s con ibu ed o he a icle and app o ed he
submi ed e sion.
FUNDING
This s udy was unded in pa by a Fund Managemen Ag eemen
be ween he IEO and he Minis e io pa a la T ansición Ecológica
(Spain) and by g an numbe IN607A2018/2 o he Axencia
Galega de Inno ación (GAIN, Xun a de Galicia, Spain).
ACKNOWLEDGMENTS
We acknowledge he Na ional Pa k o he A lan ic Islands
(Spain) o acili a ing he sampling, and Ma ia Lema (Se icio
de Análisis Ins umen al, Uni e sidad de A Co uña, Spain) o
he iso opic de e mina ions.
SUPPLEMENTARY MATERIAL
The Supplemen a y Ma e ial o his a icle can be ound
online a : h ps://www. on ie sin.o g/a icles/10.3389/ ma s.
2021.657131/ ull#supplemen a y-ma e ial
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Con lic o In e es : The au ho s decla e ha he esea ch was conduc ed in he
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po en ial con lic o in e es .
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